This thesis discusses the use of silicon as a base material for solar-to-fuel (S2F)devices.
Silicon is an attractive semiconductor material, already proven itself in a wide range of applications. ‘In photovoltaic (PV) cells and in S2F, silicon can be used. However, it is not the most obvious material to choose,’ says Wouter Vijselaar.
In a S2F devices, still other modifications are needed, as opposed to PV cells, such as: protective layers, junction optimization, anti-reflection coatings, and electrocatalysts. Furthermore, the shaping into microwires is another great improving potential to be used.
‘Combining all these interventions does certainly lead to feasible devices and promising results,’ Wouter stresses. ‘First, to enhance the efficiency, we showed that the height of the microwires should be around 40 µm. We also varied the p- and n-junction depth in the microwires. We found 790 nm to be optimal, increasing the surface area, while keeping both leaking currents and defect states under control.’
The efficiency of Si microwire cells was further improved by the addition of passivation and anti-reflection layers. ‘Al2O3 gave the highest increase in PV efficiency,’ Wouter says. ‘However, after fabricating, experimenting and characterizing other layer materials, I prefer silicon nitride (SiNx). Herewith, a photocathode that performs well can be fabricated, in which the Si microwires prove themselves stable, even in harsh alkaline electrolytes. We showed it possible to produce such a photocathode efficiently, solely using earth-abundant materials.’
Studying the three themes of his PhD work on silicon based S2F cells (microwire geometry, junction depth and coatings), Wouter intuitively followed his personal approach.
‘I like to stack well-established solutions found in literature, by combining them creatively,’ he says. ‘This approach is somewhat fallen into disuse in modern physics, I believe.’
As an example Wouter mentions his search for catalysts, necessary to enhance silicon functionality.
‘Pt is one possible, and good, solution. But this is a rare-earth material,’ he says. ‘Therefore, I opted for nickel molybdenum, combining it with the right microwire fabrication recipes and coating techniques. That is the way I like to work: not analysing all parameters into detail - looking for the best reasoned solution - but starting a novel approach, and work towards optimization. That is one of the main personal results of the PhD project. I now dare to pursuit my own ideas. My supervisor, Professor Jurriaan Huskens, always was encouraging me to do so.’
To generate enough photovoltage - to perform the required fuel production - silicon heterojunction devices were fabricated, on Si microwire arrays with a height up to 20 µm.
‘It was a great challenge to fabricate two types of multi-junctions, generating 1.4 – 1.9 Volt, sufficient for solar fuel production,’ Wouter says. ‘Because the proposed configuration is versatile, catalyzed reactions are not limited only to H2 formation as a fuel; also other electrodeposited catalysts can potentially be used for different reaction products.’
At last, in this PhD work, a membrane-embedded microporous PEC (photoelectrochemical) cell architecture was designed, fabricated and experimentally validated, that circumvents the ionic transport losses, the formation of a pH gradient, and gas crossover.
Wouter is positive that S2F cells will be feasible in future, but still further research is required. ‘New types of earth-abundant semiconductors need to be found and investigated,’ he concludes.
‘This is needed to expand the possibilities of semiconductor combinations (i.e., photocathode and -anode) in full S2F devices. However, this thesis offers new perspectives for the development of stabilized silicon microwire systems with tailored functionalities, to be applied in photoelectrochemical and electrochemical conversion processes. The microwire (or: micropillar structure) strategy has proven its industrial potential. Future applications will result from that, I am sure.’
Wouter enjoyed collaborating with other Groups. ‘Definitely for the required micro-fabrication, the expertise of Meso Scale Systems Goup was essential (i.e. Pieter Westerik and Erwin Berenschot), as were the Nanolab skills of Roald Tiggelaar.’
After his Defence, Wouter is planning to pursue a career in industry. ‘In my opinion an R&D job in a middle-sized company suits me best,’ he says. ‘The financial scope there will be broad enough to guarantee interesting R&D projects, I expect. In a multinational R&D team, the work as a chemist might be too much centred around parameter research, I am afraid. The year to come, however, I will be traveling, driving around in a self-built fire truck camper. That will give me some extra time to think.’